Various methods for tracking the point of observation, eye position, or point of gaze (which may be regarded as equivalent end-results) have been proposed but commonly these approaches suffer from complexity and consequent high cost, bulkiness or poor performance. Prior methods include attempting to track the muscular movements of the eye with mechanical transducers and various optical techniques.
The optical methods used in the prior art fall into two categories, viz, 1: tracking the position of the pupil; and 2: tracking a highlight from the cornea.
The pupil tracking systems are limited in accuracy by several factors the most important being the poor contrast between the iris and the pupil, particularly with dark coloured irises, which reduces the resolution with which a camera can define the pupil boundary. Secondly the iris commonly has an irregular dark border at the edge of the pupil which further degrades accuracy. Thirdly the relative motion of the pupil is not related to an external reference physically aligned with the user's head and so extrapolation of the pupil position to determine the point of observation yields poor repeatability.
The corneal highlight systems suffer from high degree of variation in corneal profiles both between individuals and from time to time in the same individual due to variations in eyeball pressure due to air pressure. The nature of the corneal highlight apparatus also causes further variations and poor repeatability since small changes in the relationship of the illumination source to the cornea drastically affect the position of the highlight.
Preferred embodiment of the present invention aim to obviate or overcome disadvantages of the prior art, whether referred to herein or otherwise.